Voltage source inverter based variable frequency drives (VFDs) have an AC to DC rectifier unit with a large DC capacitor to smooth the voltage ripple. In all VFDs, the DC bus voltage is inverted to three-phase variable voltage, variable frequency output to control the speed and torque of three-phase AC motors. For loads rated above 2 hp, it is customary and often required to power up the AC to DC rectifier section from a three-phase AC source. However, the input AC to DC rectifier can be powered up from a single-phase AC source, especially in locations where three-phase AC power is unavailable due to logistics and other reasons. In such cases, some utilities allow three-phase VFDs to be powered from a single-phase AC source provided the peak current flowing into the AC to DC rectifier system is within the rating of the single-phase AC source. Many VFD manufacturers impose restrictions on the rating of the VFD when they are subject to a single phase AC source.
There exist important concerns while operating a three-phase VFD from a single-phase AC source. These include that the RMS value of the input AC current is at least two times that when a three-phase supply is used for a given load. The input diodes have to handle the higher demand current for a given load. De-rating of the inverter is undertaken to address this concern. Higher input current affects the input AC power terminal blocks. In many cases, the diodes may be able to handle the higher values of RMS current but the terminal blocks may not be rated to handle the peak current on a continuous basis. Input harmonic distortion is high when single-phase input is used as an AC source for a three-phase inverter. Poor harmonics is also associated with lower input power factor. This affects the efficiency of power conversion and should be considered in any proposed improvements. Single-phase AC supply results in higher ripple voltage across the DC bus. Higher ripple voltage translates to higher ripple current through the capacitor and more heating of the capacitor. The inverter is typically de-rated to handle the higher ripple current. The current drawn from the single-phase AC source feeding a three-phase VFD is discontinuous. When the pulsed current flows from the AC source, it creates voltage drop that mimics the pulsed current waveform to some extent. The resulting voltage drop can affect other loads connected to the same AC source.
There are many known techniques, both passive and active, that are employed to improve the current waveform and reduce the overall current harmonics. The active techniques have an advantage over the passive techniques in size and performance. The cost of certain type of active techniques can be higher than passive techniques.
One known passive approach creates a resonant circuit across the DC bus. Energy is stored in the resonant components and released naturally at the appropriate time to support the sagging DC bus voltage and thereby reduce the ripple across the bulk capacitors of the DC bus. The topology is shown in FIG. 1 and its operation is described below.
The DC bus resonant method adds a resonant circuit across the DC bus as shown in FIG. 1. The resonance frequency of the resonant inductor L1 and resonant capacitor C1 of the resonant circuit across the DC bus is two times the supply frequency (100 Hz in Asia, Europe and Africa, and 120 Hz in North and South America). A DC link inductor L2 is placed in between the resonant circuit and the main bus capacitor C2 to decouple the main DC bus capacitor from the resonant circuit so as not to detune the main resonant circuit. It also helps in reducing the current ripple of the bus capacitor C2. The value of L2 is about ⅓ the value of L1 and the value of C1 is typically ⅓rd to ¼th fourth the main capacitor, C2.
When the instantaneous voltage across the resonant circuit L1-C1 is lower than the line-line voltage of the AC supply, the input diodes conduct and charge the resonant circuit L1-C1 and the DC bus filter capacitor C2. Because of the resonant circuit, the input diodes remain in conduction for one-half of the resonant cycle, even when the voltage across the resonant circuit L1-C1 is higher than the instantaneous input line-line voltage. The input diode conduction duration is extended because the charging current keeps flowing into the resonant circuit and into the main DC bus capacitor C2.
When the resonant current tries to reverse its polarity and flow in the opposite direction, the input diodes are shut off and they cease to conduct. The energy stored in the resonant circuit L1-C1 is transferred to the load thereby reducing the ripple current through the DC bus capacitor C2. This action limits the DC bus ripple voltage and extends the life of the DC bus capacitor C2. During this time, the DC bus filter capacitor C2 also discharges into the load, which is evident from the drooping DC bus voltage characteristics. The value of L1, C1, and L2 are chosen such that the resonant period when the input diodes are conducting and charging L1-C1 path is similar to the resonant period when the input diodes are not conducting and the energy in L1-C1 is being transferred to C2 via L2. Advantages of the DC bus resonance method are that extending the diode conduction period during the charging cycle reduces the input harmonics and improves the input power factor to some extent, and the DC bus capacitor ripple is significantly reduced.
The passive resonant circuit shown in FIG. 1 has disadvantages as well. Resonant components are bulky and expensive. The DC bus circuit needs to be accessed in some VFDs that do not have a built-in DC link choke, L2. The peak diode current is reduced but the improvement is not conspicuous. Also, average DC bus voltage is still low and the VFDs need to be de-rated though the level of de-rating is smaller than that without the DC bus resonant circuit.
To improve the overall performance, an active solution commonly used in single-phase AC to DC power supplies uses a boost converter that boosts the input voltage to a desired DC bus voltage level under all load conditions. The overall DC bus voltage ripple is also reduced. An added advantage is that the input current is made continuous which reduces the input current harmonic distortion. The active circuit is shown in FIG. 2.
When the switch S1 is turned ON, current from the AC source flows into the inductor L1. During this period, energy is stored in the inductor L1. When the switch S1 is turned OFF, the inductor current cannot stop flowing immediately because of the nature of inductance. The voltage across the inductor L1 forward biases the blocking diode and all its stored energy is transferred to the DC bus capacitor and the load. The switching ON and OFF of the switch S1 takes place very rapidly in kHz range and hence the output DC bus voltage is effectively regulated thereby reducing both the ripple voltage and the ripple current through the capacitor.
As shown in FIG. 2, the boost converter requires a DC link inductor for the boosting action, a switch, and a blocking diode. The main DC bus capacitor is part of the load. The control circuit takes care of the operation of the boost converter by switching the IGBT switch. The schemes developed for the control of the boost converter topology involve maintaining a regulated DC bus voltage with low ripple and simultaneously enabling unity-power-factor operation with continuous input current for the entire load range as mentioned earlier.
The relationship between the output and input voltage can be derived as follows. The output voltage is a function of the duty cycle ‘α’ of the switch. The relationship between the average output voltageVDC and the average input voltageV1 in terms of the duty cycle ‘α’ is derived next. If the switch S1 is ON for a timetON and has a cycle duration of T, then duty cycle ‘α’ is defined as:
                    α        =                                                           t                        ⁢            ON                    T                                    (        1        )            
Since the average voltage across the inductor is zero, the following expression is true:
                              (                      α            ⁢                                                  ⁢                          T              ·                              V                1                                              )                +                  (                                                    (                                  1                  -                  α                                )                            ⁢                              T                ·                                  (                                                            V                      1                                        -                                          V                                              D                        ⁢                                                                                                  ⁢                        C                                                                              )                                                      =                                          0                ⁢                                                                  ⁢                α                            =                              1                -                                                      V                    1                                                        V                                          D                      ⁢                                                                                          ⁢                      C                                                                                                                              (        2        )            
The average output voltage VDC in terms of the average input voltage Vg and duty ratio ‘α’ is given as:
                              V                      D            ⁢                                                  ⁢            C                          =                              V            1                                1            -            α                                              (        3        )            
When the duty ratio ‘α’ is zero, then the average output voltage is the same as the average input voltage because the switch S1 is not being utilized. If the duty ratio ‘α’ is 1, then the output voltage can theoretically go to infinity, which is impractical.
Average DC bus voltage control philosophy is employed. A set voltage reference is compared with the DC bus voltage that is actually sensed from across the DC bus. The error is fed through a proportional-integral (PI) controller. The output of the PI controller forms the gain, which is multiplied by the non filtered rectified voltage at the output of the single-phase rectifier. The shape of this gain manipulated rectified voltage is the desired inductor current. The DC link current through the boost inductor is also sensed and is compared with this current reference. The error is fed through another PI controller, the output of which is compared with a high frequency saw-tooth carrier frequency waveform. The comparator has a built-in (non-adjustable) hysteresis. The output of the hysteresis comparator is fed into the gate driver. The output of the gate drive circuit is used to control the switching action of the boost IGBT.
From the discussions thus far, the advantages and disadvantages of the boost converter can be summarized as follows. The input current is sinusoidal and has low harmonic distortion, thereby eliminating the peak current stress in the input diodes. The input power factor is very good and results in lower thermal loss in the AC system. The ripple voltage across the capacitor is reduced but still is conspicuous. The boost switch (S1 in FIG. 2), has to be rated to carry peak of input current and also has to handle the boosted voltage. Hence, the stress across the switch S1 is large making it expensive. Switching noise is observed in the input AC voltage waveform. This means that input EMI filter is needed to limit noise from propagating into the AC source. No de-rating of VFD is needed and VFD can be operated at its rated power condition.
The present invention is directed to improvements in single-phase front end rectifier systems.